1. Field of the Invention
The present invention generally relates to digital voice communications systems and, more particularly, to a line spectral frequency vector quantizer for code excited linear predictive (CELP) speech encoders which is efficient in terms of bits employed, robust and effective in terms of performance across speakers and handsets, moderate in terms of complexity, and accommodates effective and simple built-in transmission error detection schemes. Such devices are commonly referred to as "codec" for coder/decoder. The invention has particular application in digital cellular networks but may be advantageously used in any product line that requires speech compression for communications.
2. Description of the Prior Art
Cellular telecommunications systems in North America are evolving from their current analog frequency modulated (FM) form towards digital systems. A standard that uses a full rate 8.0 Kbps vector sum excited linear prediction (VSELP) speech coder, convolutional coding for error protection, differential quadrature phase shift keying (QPSK) modulations, and a time division, multiple access (TDMA) scheme has been adopted by the Telecommunication Industry Association (TIA). This is expected to triple the traffic carrying capacity of the cellular systems. In order to further increase its capacity by a factor of two, the TIA has begun the process of evaluating and subsequently selecting a half rate codec. For the purposes of the TIA technology assessment, the half rate codec along with its error protection should have an overall bit rate of 6.4 Kbps and is restricted to a frame size of 40 ms. The codec is expected to have a voice quality comparable to the full rate standard over a wide variety of conditions. These conditions include various speakers, influence of handsets, background noise, and channel conditions.
Codebook Excited Linear Prediction (CELP) is a technique for low rate speech coding. The basic technique consists of searching a codebook of randomly distributed excitation vectors for that vector which produces an output sequence (when filtered through pitch and linear predictive coding (LPC) short-term synthesis filters) that is closest to the input sequence. To accomplish this task, all of the candidate excitation vectors in the codebook must be filtered with both the pitch and LPC synthesis filters to produce a candidate output sequence that can then be compared to the input sequence. This makes CELP a very computationally-intensive algorithm, with typical codebooks consisting of 1024 entries, each 40 samples long. In addition, a perceptual error weighting filter is usually employed, which adds to the computational load. Fast digital signal processors have helped to implement very complex algorithms, such as CELP, in real-time, but the problem of poor quality at low bit rams persists. In order to incorporate codecs in telecommunications equipment, the voice quality needs to be comparable to the 8.0 Kbps digital cellular standard.
There are many representations of short term predictor parameters. A commonly used one is the set of line spectral frequencies. The quantization of the line spectral frequencies has been the subject of several investigations. Both scalar and vector quantizers have been designed for this purpose. Typically, the scalar quantizers need 36 to 40 bits to encode ten line spectral frequencies in order to meet the other objectives of good robust performance across speakers and handsets, moderate complexity, and built-in error detection capability. Efficiency in terms of bits needed is therefore sacrificed. Vector quantizers, on the other hand, achieve efficiency in terms of bits but at the expense of speaker or handset dependent performance and often at high complexity and without built-in error detection capability.